In this study the authors characterize peritraumatic reactions of residents of New York City during and immediately following the September 11th terrorist attacks, identify predictors of those reactions, and identify predictors of PTSD 4 months later. A cross-sectional sample of New York residents (n = 2,001) responded to questions about sociodemographics, historical factors, event-related exposure; acute cognitive, emotional, and physiological reactions to the September 11th terrorist attacks; and current (past month) PTSD symptoms. Factor analyses of peritraumatic reactions yielded three related, but distinct, peritraumatic response patterns-dissociation, emotional reactions, and panic/physiological arousal. Several demographic, historical, and exposure-related variables predicted one or more peritraumatic reaction patterns. After controlling for demographic, historical, and exposure factors, each of the peritraumatic reactions factors, one historical factor and one event-related exposure factor remained as significant predictors of PTSD. These results support a growing literature concerning the predictive value of peritraumatic reactions in relation to PTSD. Implications for preventive efforts and suggestions for future research are discussed.